[core] Add x-is-free-form vendor extension (#6849)

This adds an x-is-free-form vendor extension to allow users to skip our
"free-form" logic which would previously prevent object schemas with no
properties to be considered "free-form". The previous behavior was due
in part to Swagger Parser not exposing `additionalProperties: false` to
us (which should be similar behavior to this extension).

A free-form object is considered a dynamic object with any number of
properties/types. DefaultGenerator does not allow for generation of
models considered free-form. However, a base type with no properties and
no additional properties is allowed by OpenAPI Specification and is
meaningful in many languages (e.g. "marker interfaces" or abstract
closed types).

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -65,6 +65,8 @@ public class ModelUtils {
     // A vendor extension to track the value of the 'disallowAdditionalPropertiesIfNotPresent' CLI
     private static final String disallowAdditionalPropertiesIfNotPresent = "x-disallow-additional-properties-if-not-present";
 
+    private static final String freeFormExplicit = "x-is-free-form";
+
     private static ObjectMapper JSON_MAPPER, YAML_MAPPER;
 
     static {
@@ -672,25 +674,23 @@ public class ModelUtils {
     }
 
     /**
-     * Check to see if the schema is a model with at least one property.
+     * Check to see if the schema is a model
      *
      * @param schema potentially containing a '$ref'
      * @return true if it's a model with at least one properties
      */
     public static boolean isModel(Schema schema) {
         if (schema == null) {
-            // TODO: Is this message necessary? A null schema is not a model, so the result is correct.
-            once(LOGGER).error("Schema cannot be null in isModel check");
             return false;
         }
 
-        // has at least one property
+        // has properties
-        if (schema.getProperties() != null && !schema.getProperties().isEmpty()) {
+        if (null != schema.getProperties()) {
             return true;
         }
 
-        // composed schema is a model
+        // composed schema is a model, consider very simple ObjectSchema a model
-        return schema instanceof ComposedSchema;
+        return schema instanceof ComposedSchema || schema instanceof ObjectSchema;
     }
 
     /**
@@ -745,6 +745,16 @@ public class ModelUtils {
             // no properties
             if ((schema.getProperties() == null || schema.getProperties().isEmpty())) {
                 Schema addlProps = getAdditionalProperties(openAPI, schema);
+
+                if (schema.getExtensions() != null && schema.getExtensions().containsKey(freeFormExplicit)) {
+                    // User has hard-coded vendor extension to handle free-form evaluation.
+                    boolean isFreeFormExplicit = Boolean.parseBoolean(String.valueOf(schema.getExtensions().get(freeFormExplicit)));
+                    if (!isFreeFormExplicit && addlProps != null && addlProps.getProperties() != null && !addlProps.getProperties().isEmpty()) {
+                        once(LOGGER).error(String.format(Locale.ROOT, "Potentially confusing usage of %s within model which defines additional properties", freeFormExplicit));
+                    }
+                    return isFreeFormExplicit;
+                }
+
                 // additionalProperties not defined
                 if (addlProps == null) {
                     return true;

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java

@@ -927,6 +927,35 @@ public class JavaClientCodegenTest {
         );
     }
 
+    @Test
+    public void testAllowModelWithNoProperties() throws Exception {
+        File output = Files.createTempDirectory("test").toFile();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setLibrary(JavaClientCodegen.OKHTTP_GSON)
+                .setInputSpec("src/test/resources/2_0/emptyBaseModel.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(clientOptInput).generate();
+
+        Assert.assertEquals(files.size(), 47);
+        TestUtils.ensureContainsFile(files, output, "src/main/java/org/openapitools/client/model/RealCommand.java");
+        TestUtils.ensureContainsFile(files, output, "src/main/java/org/openapitools/client/model/Command.java");
+
+        validateJavaSourceFiles(files);
+
+        TestUtils.assertFileContains(Paths.get(output + "/src/main/java/org/openapitools/client/model/RealCommand.java"),
+                "class RealCommand extends Command");
+
+        TestUtils.assertFileContains(Paths.get(output + "/src/main/java/org/openapitools/client/model/Command.java"),
+                "class Command");
+
+        output.deleteOnExit();
+    }
+
     /**
      * See https://github.com/OpenAPITools/openapi-generator/issues/6715
      */

modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java

@@ -128,6 +128,15 @@ public class ModelUtilsTest {
         Assert.assertEquals(unusedSchemas.size(), 0);
     }
 
+    @Test
+    public void testIsModelAllowsEmptyBaseModel() {
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/2_0/emptyBaseModel.yaml");
+        Schema commandSchema = ModelUtils.getSchema(openAPI, "Command");
+
+        Assert.assertTrue(ModelUtils.isModel(commandSchema));
+        Assert.assertFalse(ModelUtils.isFreeFormObject(openAPI, commandSchema));
+    }
+
     @Test
     public void testReferencedSchema() {
         Schema otherObj = new ObjectSchema().addProperties("sprop", new StringSchema()).addProperties("iprop", new IntegerSchema());

modules/openapi-generator/src/test/resources/2_0/emptyBaseModel.yaml

@@ -0,0 +1,71 @@
+swagger: "2.0"
+info:
+  title: Test Command model generation
+  description: Test Command model generation
+  version: 1.0.0
+host: localhost:8080
+schemes:
+  - https
+definitions:
+  Command:
+    title: Command
+    description: The base object for all command objects.
+    type: object
+    # Explicitly avoid treating as a "free-form" or dynamic object, resulting in classical languages as a class with no properties.
+    x-is-free-form: false
+  RealCommand:
+    title: RealCommand
+    description: The real command.
+    allOf:
+      - $ref: '#/definitions/Command'
+  ApiError:
+    description: The base object for API errors.
+    type: object
+    required:
+      - code
+      - message
+    properties:
+      code:
+        description: The error code. Usually, it is the HTTP error code.
+        type: string
+        readOnly: true
+      message:
+        description: The error message.
+        type: string
+        readOnly: true
+    title: ApiError
+parameters:
+  b_real_command:
+    name: real_command
+    in: body
+    description: A payload for executing a real command.
+    required: true
+    schema:
+      $ref: '#/definitions/RealCommand'
+paths:
+  /execute:
+    post:
+      produces: []
+      operationId: executeRealCommand
+      parameters:
+        - name: real_command
+          in: body
+          description: A payload for executing a real command.
+          required: true
+          schema:
+            $ref: '#/definitions/RealCommand'
+      responses:
+        '204':
+          description: Successful request. No content returned.
+        '400':
+          description: Bad request.
+          schema:
+            $ref: '#/definitions/ApiError'
+        '404':
+          description: Not found.
+          schema:
+            $ref: '#/definitions/ApiError'
+        default:
+          description: Unknown error.
+          schema:
+            $ref: '#/definitions/ApiError'